1. Field of the Invention
The present invention pertains to airfoils for aircraft and the like. More particularly, the present invention relates to the construction and function of airfoils for improved performance, and finds particular application to the improvement of conventional airfoils by way of the construction and function of the trailing flap of such an airfoil.
2. Brief Description of Prior Art
The functioning of an airfoil, such as an aircraft wing, is affected by the airfoil's maximum thickness, thickness distribution, camber and camber distribution, Mach and Reynolds numbers, and the extent of laminar flow over the airfoil surfaces. A significant development in airfoil construction occurred with the development of the supercritical airfoil, designed for more efficient transonic flight. The supercritical airfoil can be designed for a given lift coefficient and thickness with a specific upper surface contour having lower drag at transonic speeds than that of previous airfoils. Generally, supercritical airfoils have larger leading edge radii and less surface curvature in the mid-chord region of the upper surface than earlier, conventional airfoils. The chord of a structure such as an airfoil or a flap is the maximum distance between the trailing edge of the structure and the foremost point on the leading edge of the structure. Supercritical airfoils are also characterized by substantial camber and low thickness near the trailing edge of the airfoil.
Various proposals to improve the performance of supercritical airfoils, such as to reduce drag at higher Mach numbers, have been made. These included making the trailing edge thicker by adding material to the lower surface, which also resulted in an increase in camber. In general, increasing the trailing edge thickness increases the drag generally without dependence on Mach number, but also produces a decrease in drag due to increased aft camber which can be sensitive to changes in Mach number. Thus, at some Mach number, the reduction in drag due to the camber effect outweighs the increase in drag due to the increased thickness of the trailing edge. If camber is increased excessively even without adding thickness to the trailing edge of a supercritical airfoil, the expected reduction in drag may be offset by increased drag due to flow separation near the trailing edge.
The primary mechanism that produces a reduction in drag with increased aft camber is the ability of the airfoil to achieve the same lift at a lower angle of attack with such increased camber; further, the lift is more evenly distributed over the chord, particularly for conventional, that is, nonsupercritical, airfoils. The lower negative pressure on the upper surface reduces the strength of any shocks and the attendant-wave drag. Additionally, a reduction in the adverse pressure gradient over the forward and middle portions of the airfoil minimizes flow separation, and thus reduces pressure drag.
Another approach used to enhance the performance of both conventional and supercritical airfoils involved adding a blunt wedge to the underside of the airfoil trailing edge, thereby increasing the thickness of the trailing edge as well as the camber at that point of the airfoil. The wedge may be constructed such that the thickness of the airfoil actually increases approaching the trailing edge. Variations in the relative shape and length of the wedge change the amount of effective trailing edge camber as well as the thickness of the trailing edge. Such a modification causes a drag reduction at mid-to-high section lift coefficients at Mach numbers approaching the drag rise Mach number.
Another approach to increasing the camber of an airfoil is to rerig the trailing edge flap element and/or aileron, by generally tilting such element downwardly to the rear of the airfoil.
In yet another approach to enhancing the performance of a conventional airfoil, camber was added to the trailing edge flap to increase the lift for a given angle of attack by making the underside surface toward the rear of the trailing edge flap concave. At the same time, the leading edge radius of the airfoil was increased to reduce the possibility of a separation bubble occurring at that location at high lift coefficients and to provide a more uniform lift distribution. Further, the top surface toward the rear of the trailing edge flap was made concave. This approach was modified by making the rear portion of the upper surface of the trailing edge flap straight, and then finally convex, in each instance further increasing the camber of the trailing edge flap and reducing the trailing edge thickness, compared to the flap with a concave upper surface, with the result of increasing lift and decreasing drag.
None of the aforementioned efforts to enhance the performance of conventional and supercritical airfoils established that a thick trailing edge on an airfoil can have less drag than a sharp trailing edge with the same camber. For structural and safety reasons, the trailing edge of wings always have a small, but finite, thickness. A sharp trailing edge is considered herein as one with a thickness on the order of one-tenth of one percent of the chord or less.
Airplanes designed twenty or thirty years ago do not have the same level of aerodynamic technology utilized in airplanes currently being constructed. FIGS. 1-3 illustrate contours of conventional airfoils typical of those employed in the 1950's and 1960's for commercial transport airplanes. Such conventional airfoils are similar to those utilized on several fighters of the Word War II era and on a number of military aircraft from the post war years as well. Such airfoils, such as illustrated in FIGS. 1-3, have a smaller nose radius than most present day supercritical airfoils, and a larger wedge angle at the trailing edge as well. Almost no camber is present over the last thirty or forty percent of the chord. Many airfoil sections in common use in the 1960's and 1970's have such trailing edges, with only modest contouring of the upper and/or lower surface, providing only small amounts of camber near the trailing edge.
It would be advantageous and desirable to provide a practical modification to conventional airfoils, for example, to increase their performance parameters to more closely approximate performance of supercritical airfoils, for example. It is an object of the present invention to provide such an improvement in the design of those airfoils. It is a further object of the present invention to provide a practical improvement in the construction of conventional airfoils, and even more modem technology airfoils, to enhance performance characteristics of such airfoils. It is a further object of the present invention to provide an improvement to airfoils to reduced their cruise drag and increase their lift capability for a given angle of attack.